The present invention is directed to a method for manufacturing a chalcopyrite solar cell.
A conversion of light into electrical power with solar cells must become decisively cheaper in order to complete with conventional methods of power generation. In addition, high-efficiency solar cells having an efficiency of at least 15% are required in order to limit the relatively high surface area requirements for the exploitation of solar energy.
Conventional solar cells are usually composed of monocrystalline or polycrystalline silicon and, recently, also include amorphous silicon to an increasing degree. However, these types of solar cells have many disadvantages. For example, solar cells of a crystalling or polycrystalling silicon are expensive to manufacture and solar cells using the amorphous silicon have an inadequate stability and have not yet achieved an adequate efficiency.
What is referred to as chalcopyrite solar cells whose crux is formed by an absorber layer having a I-II-VI.sub.2 compound semiconductor are the subject matter of recent investigations. A functional solar cell is obtained in combination with a suitable window layer composed of a lattice-matched semiconductor material as the emitter and surface passivation layer.
A solar cell having a copper indium diselenide (CIS) as an absorber is disclosed in an article by K. Mitchell et al, "Single and Tandem Junction CulnSe.sub.2 Cell and Module Technology", IEEE, 1988, pp. 1384-1389. The semiconductor layer is applied in a polycrystallling form on a glass substrate coated with molybdenum as a back side electrode. A thin, n-conductive cadmium sulfide layer is applied over this p-conductive CIS layer and serves as an electron emitter and a zinc oxide layer serves as a transparent electrode. A grid conducts the current generated with this hitherto highest efficiency of 14.1% out of the cell.
A great problem in the manufacture of a chalcopyrite solar cell is to find a material suitable as a window layer. In order to produce the junction between the absorber and the window layer that has only few traps, high demands are made on the physical properties of the window material, whose crystal lattice constant cannot deviate from the one of the absorber by more than 1. The element cadmium in the window layer presents additional problems including environmental incompatibility thereof and requires special safety precautions during production, employment and disposal of the solar cells. Moreover, the indium constituent in the known CIS cell, which have the hitherto highest efficiency, is a relatively rare element so that bottlenecks in terms of material and an increase in price can be anticipated, given expansion of the production of these types of cells.